The cellular factors which regulate MuLV production by infected cells are being analyzed. One approach is to compare MuLV production by Friend leukemia cells induced to synthesize hemoglobin with uninduced control cells. Another approach is to analyze MuLV formation in synchronously growing and in Go arrested cells. Recently, we have studied extensively the MuLV envelope glycoproteins. The causes for glycoprotein size and charge heterogeneity are being analyzed. In addition, studies are continuing on two MuLV mutants with altered envelope glycoprotein genes. One is a deletion mutant of Friend MuLV and the other is a temperature-sensitive mutant of Rauscher MuLV. These mutants are being thoroughly studied.